This invention relates to a method for separating bitumen from bituminous sand using an aqueous liquid. In particular, the invention concerns a method for preventing contamination of process water with finely divided solids during separation of bitumen from solids.
Bituminous sand from the Canadian Province of Alberta is made up primarily of a mixture of particulate solids, bitumen and water. Sand grains are each contained within a film of water. An envelope of bitumen surrounds the wetted grains. The interstices between sand particles contain bitumen, water, gas and a substantial amount of very finely divided inorganic solid material such as clay and silt. Clay and silt often make up as much as 10-30% of the tar sand.
Typically, separation of bitumen from particulate solids using a high-temperature, aqueous liquid involves (1) "conditioning", in which the raw bituminous sand is mixed with steam, liquid water, and usually also with surfactants and/or frothing agents, to form a pulp, and the water-sand pulp is adjusted to the desired consistency and temperature, and a slightly basic pH and screened and passed to a separation cell or zone, which contains a relatively large body of aqueous liquid; (2) "primary separation", in which the sand and other particulates settle out to the bottom of the aqueous liquid phase as a bottoms layer, the essentially water-insoluble bitumen floats on the top of the aqueous phase as a froth, the froth is separated for further processing, the sand is discharged as tailings, the aqueous liquid phase or "middlings" is split into a recycle stream, which passes back to the conditioning step, and a drag stream; (3) "scavenging", or "secondary separation", in which a further quantity of bitumen is recovered from the aqueous drag stream by air-induced froth flotation in a secondary cell, where the drag stream is collected as a relatively large body of aqueous liquid. The aqueous liquid effluent from the scavenging step is usually directly discharged from the processing system, because it is too heavily contaminated with dispersed, finely divided solids for further use. Bitumen obtained from the secondary separation step is normally mixed with that recovered in the primary separation step. Bitumen separated from the particulate, inorganic solids in the general manner described above is normally further purified and then subjected to one or more or refining operations analogous to those used for refining petroleum, such as fractionation and coking.
The conditioning step in bitumen separation usually involves heating the bituminous sand to an elevated temperature, e.g., 65.degree.-95.degree. C., but such heating is not for the purpose of dehydrating the sand, and typically takes place in the presence of liquid water, which is added in order to provide a pulp of the desired consistency. The heating conventionally carried out in the conditioning step is primarily to give the water-tar sand mixture, or pulp, the desired consistency, both by softening the bitumen and by decrepitating the larger lumps of tar sand. A discussion of the make-up of bituminous sand and of conventional processes for separating bitumen is included in the Synthetic Fuels Data Handbook, Thomas A. Hendrickson, Editor (1975), the complete disclosure of which is hereby incorporated in this specification.
Separation of bitumen from bituminous sand by contact with an aqueous liquid phase is not invariably carried out at higher temperatures. In the "cold-water" type separation procedures, bituminous sand is typically first mixed with a relatively lower-boiling, non-viscous bitumen solvent or diluent, such as naphtha, and this mixture is contacted with an aqueous liquid, typically at a temperature below 65.degree. C. The mixture or solution of bitumen and solvent forms a separate, oily phase which floats on the aqueous liquid and is skimmed off the aqueous phase. The particulate inorganic materials settle to the bottom of the aqueous liquid phase.
The water used in bitumen separation in commercial aqueous-phase separation operations, such as middlings or scavenging water, rapidly becomes heavily contaminated with finely divided solids, particularly clay. When the concentration of fine solid contaminants becomes too high, the contaminated water can no longer be practicably used in the aqueous liquid separation operation, because the finely divided solids seriously retard separation of the bitumen from the aqueous liquid. The fine, solid contaminants are present in the process water as a stable dispersion, and do not settle out easily when the process water is allowed to stand without agitation. The solids-contaminated water must be segregated from ground water, both because of its solid content and also because the processing streams are often contaminated with residual bitumen. The waste water segregation requirement necessitates the use of extremely large storage ponds where the contaminated water is held for long periods of time. In commercial aqueous-phase bitumen recovery operations, storage of large amounts of solids-contaminated water presents serious economic and environmental problems. Some of the problems associated with water-solids dispersions in tar sands processing are recognized and discussed in, for example: U.S. Pat. No. 3,953,318, which suggests recycling sludge to reduce the amount of stored sludge; U.S. Pat. No. 4,008,146, which suggests mixing sand with sludge to aid in settling solids; and U.S. Pat. No. 4,018,664, which suggests mixing sludge with diluent water to reduce the settling problem.
A heating step has been included in various systems proposed for treating bituminous sand. In the "hot-water" type separation system shown in U.S. Pat. No. 4,067,796, the conditioning step involves heating bituminous sand in the presence of liquid water. Heating is used to retort and disintegrate bituminous sand in a process disclosed in U.S. Pat. No. 1,592,179, whch describes heating bituminous sand with superheated steam and a hydrocarbon gas to draw off vaporized bitumen.